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Private Set Intersection

Problem

Two or more parties need to learn which records overlap without revealing nonmatching records.

When To Use

Use PSI for cohort matching, fraud signals, contact discovery, and measurement joins.

When Not To Use

Avoid PSI if revealing the intersection is too sensitive or repeated matching enables profiling.

Typical Architecture

Parties encode records, run a PSI protocol, and reveal the intersection or aggregate statistics over the intersection.

Threat Model

Commonly honest-but-curious parties. Malicious variants are needed when parties may submit malformed inputs.

Privacy Properties

Nonmatching elements are hidden under protocol assumptions. The intersection and its size may still leak sensitive information.

Tools And Building Blocks

OPRFs, ECDH PSI, circuit PSI, MPC for post-intersection computation, and output policies.

Common Failure Modes

Identifier normalization leakage, frequency leakage, small intersections, replayed queries, and unclear deletion policies.

Open Research Problems

Better PSI developer experience, malicious-secure protocols at production scale, and safer post-PSI analytics.

MPC, clean rooms, advertising use cases, collusion.